1. Field of the Invention
The field of art to which this invention pertains is the reductive removal of a nitrogen oxide from a gaseous stream. More specifically, this invention relates to such reductive removal characterized by using ammonia as the reducing agent and a mixture of catalysts, each having specific metals or compounds of metals supported thereon.
2. Description of the Prior Art
The effective removal of a nitrogen oxide contained in gaseous streams, particularly waste streams, from various combustion systems has become an important task in view of heightened public awareness of the problems and effects of air pollution. In rising to this task, a wealth of art has recently been published dealing specifically with nitrogen oxide removal. Examples of these publications are: U.S. Pat. Nos. 4,093,561; 4,096,096; 4,117,081; 4,119,568; and 4,119,703.
A common teaching throughout the above publications is that a nitrogen oxide may be reductively removed from a gaseous stream by contacting the stream with a reductant such as ammonia in the presence of a supported metallic catalyst. Following is a tabulation of the above publications and respective catalysts disclosed by each:
______________________________________ Publication U.S. Pat. No. Catalyst ______________________________________ 4,093,561 Calcined iron hydroxide coated with alumina. 4,096,096 Calcined precipitate of a solution of aluminum and iron containing substances. 4,117,081 Mixture of copper, rhodium, and one of the metals potassium, barium, magnesium, chromium, manganese and molybdenum on alumina. 4,119,568 Alumina or silica-alumina carrier supporting a compound or mixture of com- pounds selected from the group consisting of oxides, sulfates and phosphates of a first transition metal of the Periodic Table. 4,119,703 An oxide or sulfate of iron or copper, or mixtures thereof on a silica-alumina carrier. ______________________________________
I have discovered an extremely effective process for the reductive removal of nitrogen oxides from a gaseous stream utilizing a specific mixture of catalysts not heretofore known to the art.